A GFCI is a device that is capable of switching between a tripped (open) and an operative (closed) condition based on the detection of selected criteria. Specifically, a GFCI device is designed to interrupt the supply of electric power when the device detects that current is traveling along an unintended path (e.g., through a person, or through water, etc.). GFCI devices may be included in any of numerous types of components that are capable of interrupting the supply of electric power, such as arc fault interrupters, circuit breakers, electrical outlets, etc.
GFCI outlets and circuit breakers have become widely used throughout the United States and are credited with saving many lives. Although the widespread use of GFCI devices for the past thirty-plus years has led to a large number of installations, these devices are susceptible to deterioration and eventual failure. Failure of the GFCI device can lead to the device providing electrical power like any normal circuit breaker or outlet, even though the protective features that differentiate the GFCI device from conventional devices is no longer functional. This creates a dangerous situation where the GFCI device is still viewed as functional and providing life safety protection when, in fact, it is not.
Typical GFCI devices are provided with a testing feature on the face of the device. For example, for a GFCI outlet there is a “test” button and a “reset” button. When a user pushes the test button, this simulates a problem such that the outlet should toggle to a tripped or open state to interrupt the supply of electrical power to the “load” terminals and to the any device plugged into the outlet. A similar feature is provided on a GFCI circuit breaker where pressing the “test” button should interrupt the supply of electrical power to the electrical circuit that is connected to the “load” terminal of the circuit breaker. However, it has been shown that most individuals do not regularly test the GFCI outlets or circuit breakers in their homes to ensure that they are functioning correctly. Likewise, even in commercial applications, the GFCI devices are rarely tested.
This leads to the dangerous situation that GFCI devices that are currently in service are not functioning to provide the protection intended but are still in service as they continue to provide electrical power.
This potentially dangerous situation has led to the idea that GFCI devices should be self-testing which idea has been embodied in changes to the UL-943 standard in the United States. In particular, the concept of self-testing is to occur automatically and if a device is found to be defective there should be a warning to that effect that indicates the state of the device to personnel. A challenge faced by manufacturers is how to perform the self-testing automatically without impacting the normal function and the operation of the GFCI device. In other words, the GFCI device must be tested on a regular basis to determine if it is still operational, however, this self-testing cannot interfere with the tripping of the GFCI device in the event that an actual ground fault occurred.
Since June 2015, the UL-943 standard requires manufacturers to provide GFCI devices with a self-test function that automatically performs an internal test to ensure the GFCI is still functional and can properly trip during a ground fault condition. If, during the self-test, it is determined that the GFCI device is no longer functional, the device must either deny power or provide a visual and/or audible indication that the device is no longer functioning properly.
In the move from manual tested to self-testing GFCI devices, many manufacturers have sought to utilize digital technology to facilitate the self-testing functionality. However, a major problem with previously known designs is that they suffer from nuisance tripping.
U.S. Pat. No. 9,118,174 (“the '174 patent”) is directed toward a system that seeks to provide a self-testing GFCI by use of voltage level comparison and indirect sampling. In particular, the '174 patent uses a microprocessor in conjunction with a software program to perform an automatic self-test. The '174 patent uses a driver that receives a signal from the microprocessor to inject a test signal indicative of a ground fault when the self-test is performed. The '174 patent seeks to perform the automatic self-test function without interfering with the GFCI functionality and not causing false trips. This is done by looking at a value of the power indication signal generated by the fault detection circuit. However, a major drawback to the design of the '174 patent relates to the methodology of injecting the test pulse into the device.
A challenge that manufacturers face is how to inject the test pulse into the device without interfering with other signals present in the device. For example, in a GFCI circuit, there are areas of the half wave that should be avoided for the self-test signal injection. For example, if the half wave is broken into four quadrants, the GFCI charging cap occurs typically during the middle part of the second quadrant (e.g., during the rising edge but before the peak amplitude is reached). To avoid interference, the '174 teaches use of internal timers and measures a rising edge, which once a threshold level is reached, starts the timer such that the self-test signal is then injected into the half wave during the fourth quadrant (e.g., during the falling edge but before the peak reaches zero). This method is relatively effective in avoiding the charging cap in the second quadrant, however, as the signal is shifted relatively late in the fourth quadrant, there is the propensity that interference could occur with the next half wave. Likewise, the system taught in the '174 that uses the timing concept is only useable for single phase operation.